Properties of a rapidly solidified Ni–Nb layer prepared using a high-current pulsed electron beam

Highlights

• The Ni–Nb amorphous alloy layer was prepared by HCPEB irradiation.

• Amorphous and nanocrystalline were formed in the alloying layers.

• The surface composition of the alloys gradually becomes uniform.

• The microhardness and corrosion resistance were increased by HCPEB irradiation.

Abstract

In this study, the surface of a bulk crystalline Ni–Nb powder metallurgy (PM) sample of a glass-forming composition was investigated using high-current pulsed electron-beam (HCPEB) irradiation, which also improved the surface properties. Unlike the original crystalline sample, X-ray diffraction (XRD) of the surface shows an amorphous phase following HCPEB exposure. Transmission electron microscopy (TEM) reveals that, after the Ni–Nb PM samples were exposed to 20 or 30 pulses of HCPEB irradiation, the re-melted layer contains localized nanocrystalline regions within an amorphous matrix. The scanning electron microscopy (SEM) results indicate that, after 30 pulses, a re-melted layer about 5 μm in thickness forms on the surface of the Ni–Nb PM samples. Meanwhile, a large number of cracks and craters were observed on the re-melted layer. In addition, the micro-hardness tests reveal that the micro-hardness improves significantly after HCPEB irradiation. The microhardness of the sample was the highest after the 30 pulses, which was 1.5 times that of the original sample. Furthermore, potentiodynamic polarization tests indicate that the irradiated samples of corrosion resistance were not enhanced compare to the initial samples, due to the formation of cracks providing a Cl⁻ etching channel that accelerates the corrosion rate.

Introduction

Amorphous or metallic glass has become a new class of alloys that is clearly different from conventional traditional crystalline metal materials. This new class can be characterized by atomic long-range disorder but short-range order, without a spatial unit structure. As a result, these new materials have many properties, which do not exist in conventional alloys. While amorphous structures can be obtained with many alloy systems, Ni-based amorphous alloys offer very special properties, e.g. excellent corrosion resistance [1], high thermal stability [2], and broad subcooled liquid zone. Rapid cooling has already been used to produce a series of Ni-based bulk metallic glasses (BMGs) such as Ni- (Nb, Ta) [3,4], Ni–Nb- (Ta, Sn, Sb, Zr) [1,[5], [6], [7]], Ni-Zr-Nb-Al [8], and Ni–Zr–Ti–Nb–Si–Sn, [9]. However, industrial applications of bulk metallic glasses (BMGs) are restricted due to the limitations of the shape, size and preparation techniques for preparing bulk three-dimensional amorphous alloys [10,11]. Amorphous coatings, in contrast, can meet industrial requirements much easier. The lower dimension of coatings in one direction i.e. thickness, and become increasingly important for material applications in recent years. Li [12,13] et al. successfully prepared a largely amorphous Al–Co–Ce layer using large-area pulse electron beam (LAEB) irradiation and found discovered corrosion resistance. Li [14,15] et al. prepared Ni–Nb-(Zr, Mo) amorphous-crystalline multilayered films using ion-beam mixing. The amorphous layer, which was prepared via ion-beam mixing, was also found to exhibit better corrosion resistance. This indicates that using a high-energy beam, such as electron- or ion-beam with high cooling rates, facilitates the manufacturing of amorphous films or surface layers.

High-current pulse electron-beam (HCPEB) treatment is a relatively new surface modification technique. The range of vacuum condition of HCPEB equipment is 3–8*10⁻³ Pa. Under this condition, the spark source could effectively conduct to generate the plasma discharge density maintained low pressure (20-30Kv) explosive electron emission, and the diffusion velocity of plasma satisfied the uniformity of beam spots. The plasma discharge density could not be guaranteed under the higher vacuum, and the diffusion of the beam spots is non-uniform under the lower vacuum. In addition, due to the presence of residual gas, oxidation, contamination and other impurities of samples would be occurred in the lower vacuum under working conditions of HCPEB equipment, thereby reducing the quality of surface. And under sub-atmosphere ambient gas, a severe gas discharge would occur due to the high energy density (4 J/cm²), which could cause damage to HCPEB equipment once reach the breakdown voltage. Therefore, based on the above factors, the chamber is evacuated by molecular pump to a base pressure of 5 × 10⁻³ Pa, which is isolates the sample from the air and effectively prevents oxidation, contamination and other impurities in the surface layer of the sample, and avoiding damage to the equipment meanwhile achieving optimal experimental results. To be modified materials can be altered to acquire enhanced surface properties by adjusting parameters like energy injection, pulse width, and the number of irradiation exposures. An electron beam with extremely high energy (10⁷–10⁹ W/cm²) hitting the materials surface results in rapid heating/cooling and induced thermal stress. As a result, microstructures such as nanocrystals, amorphous phases, and various crystal defects can be found in the re-melted layer. This means, this surface treatment technique has a huge commercial potential. While many researchers have confirmed the formation of amorphous structures in re-melted layers following HCPEB treatment [[16], [17], [18]], only few specifically investigated the preparation of amorphous layers on the surface of the material with the aim to enhance the surface properties.

The Ni–Nb alloys system is a high glass-forming ability (GFA) system, which can produce amorphous alloys for a wide range of compositions. The best amorphous forming component of the Ni–Nb system is Ni₆₂Nb₃₈ [1,19]. In this paper, the Ni–Nb powder metallurgy (PM) samples were irradiated using HCPEB exposure to study the surface microstructure and evolution of the surface properties as a function of several parameters.